It is often useful, using established quality assurance techniques, to measure the temperature of a product by a sensing mechanism which does not actually contact the product. Presently available non-contact radiation detectors are currently used for this purpose. For example, in a given manufacturing process the proper temperature value for a product as it exits an oven may be established. It may then be desirable to monitor the temperature of the products as they leave the oven and adjust the oven accordingly if the monitored temperature deviates from the established temperature. Accordingly, non-contact radiation detectors are especially desirable in assembly line type manufacturing processes.
Non-contact infrared detectors are based on the principle that the thermal radiation emitted from a surface is proportional to the temperature of the surface raised to the fourth power. Thermal radiation detectors, such as thermopiles, sense the total thermal radiation emitted by a given target; a non-contact infrared sensing system can incorporate a thermopile and computing means to calculate a temperature value for the target based on the relationship between thermal radiation and temperature. Many of the presently available infrared detector systems are inaccurate, however, because they do not account for the fact that the temperature of the object is a function of the emissivity of the object's surface and of the other radiation which is independent of the temperature of the target. Excepting theoretical bodies at absolute zero temperature, all objects emit radiant heat from their surfaces which can include both emitted and extraneous radiation. Extraneous radiation is defined as any heat radiated from the object which is not related to the object's temperature. This includes reflected and transmitted heat. Emissivity, defined as the ratio of the actual emitted heat to the maximum theoretical emitted heat, can range from zero to unity for a given object. Therefore, to properly calculate the temperature of a given target object using a non-contact radiation detector, three variables must be known: the emissivity of the target, the value of the extraneous radiation and the total infrared radiation from the target (often expressed in British thermal units per target area over time). The well known Stefan-Boltzman equation may be used to mathematically define the interrelationship between these variables: ##EQU1## where T.sub.T =the temperature of the target in degrees Celcius, .epsilon.=the emissivity value of the target, R.sub.T =the total infrared radiation from the target in British thermal units per hour per feet squared, .sigma. is the Stefan-Boltzman constant equal to 1.816.times.10.sup.-8, and T.sub.R is the temperature of the target's surroundings reflecting off the surface of the target (extraneous radiation).
In most applications, however, the value of the emissivity of a given target object and the extraneous radiation will not be known. This is further complicated in situations where ambient conditions change, resulting in either an increase or a decrease in extraneous radiation. Using presently available non-contact radiation detector systems, if the emissivity of the target is not known then an emissivity value must be supplied to the system based on the total radiation received from the target object, which includes both the emitted and the extraneous radiation. Since the extraneous radiation is not eliminated from the reading of the total heat radiated from the target, these radiation detectors do not accurately detect the temperature of the targets, over a wide range of varying target temperatures.
Other types of infrared sensors employ a method where entered constants are used to offset and negate extraneous radiation based on two different temperature measurements, but these devices never determine an emissivity value for the target. As a result, complicated iteration processes must be used to establish a temperature value for the object. These iterations are time consuming and sometimes require complicated algorithms which may not be suitable for applications where target temperature must be determined quickly.
Another system for accommodating for the effects of emissivity and extraneous radiation uses a high reflectivity hemispherical cup placed against the surface of the target. Radiation is detected through an aperture in the cup and, by reflecting all emissions from the target back to the target, the cup causes the surface of the target to behave similar to a black body, the emissivity of which is nearly equal to unity regardless of the actual emissivity of the surface of the target. Since the cup must contact or come very close to the target, however, this system is not useful for irregularly shaped targets or situations where quick readings must be made on successive targets from a distance.
A better system would include a way to determine an emissivity value for the first of a series of targets having similar emissivity values and include means to resolve a value for the extraneous radiation. In this way, temperature values for subsequent targets could be more accurately established using the emissivity value determined for the first target, thereby eliminating the effects of extraneous radiation. At the present time, however, no such system exists.